Efficient combining of RF signals of the same or similar frequencies with varying relative phases and amplitudes cannot be achieved with such devices as Wilkinson or hybrid combiners, as they rely upon specific relative power levels and phases of the input signals to operate effectively and provide isolation between their inputs.
In the case of standard quadrature hybrid combiners (such as branch line couplers), the phase of the input signals must differ by 90 degrees with equal amplitudes for optimum combining efficiency. These criteria are required to ensure optimal voltage cancellation at specific nodes within the combining network, thus providing isolation and efficient operation.
An ultra high efficiency RF power amplifier could be realized by employing different biasing, device sizing and RF drive levels to the individual high power gain stages. However, the aforementioned combining schemes cannot provide efficient combining and isolation between the stages of such a power amplifier. Therefore, efficiency would be degraded, and portions of the RF energy from a single stage will be delivered to the other stages, effectively causing load impedance shifts, complicating the amplifier's behavior.
In the case of Wilkinson combiners, specific relative amplitudes and phases (usually zero) are required to minimize loss within the isolation resistor. Using such prior combining schemes, failure of a high power gain stage will result in a dramatic drop in efficiency. In the case where two stages are combined by a quadrature hybrid or Wilkinson combiner, at least half of the power of the stage remaining in operation will be lost, due to loss within the combining network.